1. Field of the Invention
The present invention relates to a pneumatic safety tire and more specifically to an improvement in a pneumatic safety tire which enables run-flat travel at an aspect ratio of 60% or less (a ratio of section height to the maximum width of a tire).
2. Description of the Prior Art
In addition to a strong demand for higher high-speed performance tires with increasing vehicle speed, ultrahigh performance tires which can realize spareless tire or run-flat travel have strongly been required to decrease overall vehicle weight.
Various structures of this kind of tire have been proposed under consideration of load supporting capability assured when the tire is punctured. The same applicant has already proposed a safety tire such that the inner surface of a carcass is reinforced by a crescent-shaped cross-section reinforcing layer extending from near the bead portion to the outer edge of the belt on both the sides of the tire. In the safety tire provided with a single crescent-shaped cross-section reinforcing layer, however, run-flat travel tests have clarified the following problems: cracks develop in the maximum deformation portion of the crescent-shaped cross-section reinforced layer, that is, in the inner surface of the maximum distortion portion during run-flat travel, and spread on the tire width direction to such an extent that cords constituting the carcass are cut off (called CBU) into out-of-use.
The inventors have examined the causes of the above cracks and have found that: although the elastic function of the crescent-shaped cross-section reinforced layer can be maintained at the early stage of the run-flat travel, the reinforcing rubber layer itself is deteriorated due to heat generated from inside the rubber with increasing tire travel distance, so that the elastic function of the reinforcing rubber layer is degraded without being able to support a load and therefore the degree of tire deformation increases to such an extent that cracks will be further generated.
To solve the above-mentioned problem, it is possible to prevent the deterioration of reinforcing rubber layer's elastic function due to the above-mentioned heat generation, by uniformly increasing the wall thickness of the reinforcing rubber layer. However, where the wall thickness of the reinforcing rubber layer is simply increased overall the layer, there exists another problem such that riding comfortableness is markedly degraded under the normal vehicle travel conditions.